The present invention relates to a process for the depolymerization of nylon-containing waste to form caprolactam.
Recovery of caprolactam from nylon 6 scrap (in other words, nylon 6 polymer that is substantially free of non-nylon 6 materials) has been practiced for at least twenty years. In general, nylon 6 is depolymerized by heating at elevated temperatures, usually in the presence of a catalyst and/or steam. See U.S. Pat. Nos. 4,107,160; 5,233,037; 5,294,707; 5,359,062; 5,360,905; 5,468,900; and Example 5 of European Patent Application 608,454. The caprolactam produced may be removed as a vapor stream as taught by AlliedSignal's U.S. Pat. No. 3,182,055. An extensive review of the field has been given by L. A. Dmitrieva et al, Fibre Chemistry, Vol. 17, No. 4, March 1986, pp. 229-241. Also, see U.S. Pat. No. 3,939,153.
In contrast to the depolymerization of nylon 6, nylon 66, which is substantially free of non-nylon 66 materials, is depolymerized by hydrolysis as taught by U.S. Pat. Nos. 4,578,510; 4,605,762; and 4,620,032.
U.S. Pat. No. 5,266,694 teaches that a mixture of nylon 6 and nylon 66 may be depolymerized by use of a catalyst. U.S. Pat. No. 5,310,905 teaches that a mixture of nylon 6 and nylon 66 is first separated from consumer waste, e.g. used carpet or carpet scrap, by extraction with aliphatic carboxylic acid; the filtrate comprising the acid and extracted nylon 6 and nylon 66 is then depolymerized. U.S. Pat. No. 5,241,066 teaches that a mixture of nylon 6 and PET, which is acid insoluble, is mixed with acid so that the dissolved nylon 6 may be removed from PET; the removed nylon is then depolymerized. AlliedSignal's U.S. Pat. No. 3,317,519 teaches that a yarn blend of nylon 6 and PET may be depolymerized by heating with aqueous alkali metal hydroxide at elevated pressure.
However, in the case of multi-component mixtures or composites that contain nylon 6 as one component, recovery of caprolactam is complicated by the presence of the other components. These other components and/or their decomposition products generated under conventional nylon 6 depolymerization conditions interfere with the isolation of caprolactam of adequate purity, thus necessitating expensive additional purification steps.
It would be particularly beneficial if an inexpensive method could be developed for the recovery of caprolactam from multi-component composites or materials that include nylon 6, such as carpets. The prospect of recycling such material presents a tremendous opportunity to reduce landfill usage and the costs of disposal, as well as an opportunity to reuse natural resources.
Carpets include a face fiber that is adhered to a backing (support) material which may include jute, polypropylene, latex (such as a styrene-butadiene rubber (SBR)) and a variety of inorganic materials such as calcium carbonate, clay, or hydrated alumina fillers. Nylon 6 is often used for the face fiber. Typically, carpet comprises about 20-55 percent by weight face fiber and 45-80 percent by weight backing materials. In addition, the fiber contains dyes, soil repellents, stabilizers, and other compounds added during fiber and/or carpet manufacture. Waste carpet may also contain a host of other impurities, which will collectively be referred to herein as "dirt".
These non-nylon 6 components interfere with caprolactam recovery. For example, one of the most difficult problems is that alkaline components, such as the calcium carbonate filler, neutralize acidic catalysts, such as phosphoric acid, that are conventionally used to promote nylon 6 depolymerization, thus requiring the use of increased amounts of catalyst. Another problem is that polypropylene and latex partially decompose to a mixture of hydrocarbons that co-distill with caprolactam. The remaining, partially decomposed, non-distilled portion, along with the filler and other inorganic components, renders the reaction mixture very viscous and difficult to process in conventional equipment.
Indicative of the difficulties encountered in attempting to recover caprolactam from nylon 6 carpet are the results described in U.S. Pat. No. 5,169,870 (Corbin et al.) and WO 94/06763 (Corbin et al.). In Example 1 of each publication, the crude yield of caprolactam was reported as 56% from a feedstock obtained by mechanically separating a portion of the carpet backing and subjecting the enriched nylon 6 to depolymerization; steam and 85% phosphoric acid were used in the depolymerization respectively at the rate of 33 and 0.55 parts per part of crude caprolactam produced. In Example 3 of each publication, a carpet was depolymerized without prior mechanical separation of the backing; steam and 85% phosphoric acid were used respectively at the rate of 51 and 0.30 parts per part of crude caprolactam produced. (The yield of caprolactam was not stated.) It is evident that the high expenditure of steam and phosphoric acid, and the low yield of caprolactam, render this process economically unattractive. Examples 4 and 5 of WO 94/06763 report higher yields of caprolactam, but initial separation techniques to reduce the amount of CaCO.sub.3 prior to depolymerization were required. U.S. Pat. No. 5,455,346 describes a process applicable to the recovery of caprolactam from mixtures containing nylon 6, including nylon 6 carpets. Initial separation techniques are also used to increase the nylon 6 content of the mixture prior to depolymerization; Example 13 teaches that the carpeting was freed from polyamide-free components until the polycaprolactam was 75 percent by weight based on the mixture. In contrast, it is often desirable to avoid such separation techniques.
One way to circumvent the problems associated with the presence of non-nylon 6 components in a material that includes both nylon 6 and non-nylon 6 components involves heating the waste material under pressure in water, separating the resulting solution from the non-nylon 6 components, and recovering caprolactam from the aqueous solution by further treatment. Processes based on these general principles are described in Czechoslovakian Patent No. 143,502 to Petru et al. and in AlliedSignal's U.S. Pat. No. 5,457,197 to Sifniades et al. Although these processes are an improvement, they suffer from the disadvantage of requiring multiple steps and/or high pressure operations with associated higher capital investment and operating expenses.
A need still exists, therefore, for an efficient process for recovery of caprolactam from multi-component materials that include nylon 6.